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Abstract

The sites and pathways of transpiration from leaves of Avena sterilis L. var. Algerian were studied using the accumulation of monosilicic acid as a tracer for water movement. Seedlings of Algerian oats were grown under silicon free conditions and fed monosilicic acid, in a normal nutrient solution, via the roots. The silicon component of monosilicic acid was located in freeze substituted tissue by means of x-ray microprobe analysis. Methods of tissue fixation preventing post treatment movement of tracer were developed and it was determined that monosilicic acid is a suitable tracer for water. Sites of water loss were marked by accumulation of silicon. Internal evaporating surfaces having a high intensity of water loss were demonstrated. Evaporation from epidermal surfaces was most intense over the guard and subsidiary cells with very little evaporation from the cuticular surfaces of normal epidermal cells. Moderately high evaporation occurred from epidermal fibre cells located above the veins. Evaporation from all exposed walls of guard cells including the wall adjacent to the pore was intense. Smaller amounts of tracer were located in the unexposed anticlinal walls of epidermal cells as well as within the unexposed walls of mesophyll cells. The results are interpreted as demonstrating the extent of internal transpiration surfaces and that cuticular epidermal transpiration is low. Strong support is given to the existence of peristomatal transpiration. Internal pathways of water movement are defined and the occurrence of these is discussed in relation to cuticular transpiration and lateral water movement in the epidermis.